AC to DC converters usually employ step-down transformers for reducing an AC line voltage, typically 120 volts, 60 Hertz, to an appropriate level, to enable a DC voltage having a value much lower than the AC line voltage to be derived. The transformer includes primary and secondary windings respectively connected to the AC line and across opposite terminals of an AC to DC rectifier, frequently a bridge rectifier. An electrolytic capacitor connected in shunt with output terminals of the rectifier smooths the DC voltage developed by the rectifier into a relatively low ripple DC voltage, typically applied to a DC load by way of a regulator. Frequently, output terminals of the regulator and the load terminals are shunted by an electrolytic capacitor.
In response to spikes on the AC line as frequently occur for various reasons, there is a tendency for bipolar pulses to be superimposed on the DC level applied to the DC load by the converter. The bipolar pulses are derived as a result of transient effects of the step-down transformer inductance, and the magnetic field stored in a magnetic core thereof. The bipolar pulse has an amplitude in each direction which is likely to be considerably higher than the DC level developed across the output terminals of the rectifier, and applied to the load. The pulses frequently are characterized by having a leading edge on the order of one-tenth of a microsecond, a bipolar sinusoidal like transition with a duration on the order of ten microseconds, and a trailing edge on the order of one-tenth of a microsecond. The peak-to-peak sinusoidal like variation may be as great as 50 volts, superimposed on the DC level of the rectifier and the load; typically, the DC levels are in the 5 volt range, whereby the bipolar pulse has maximum amplitudes of 30 volts on one side of ground and 20 volts on the other side of ground.
The short duration leading and trailing edges of the bipolar pulses include very high frequency components. The edges have such high frequencies that the electrolytic capacitor does not respond to them as a capacitor, but responds to them as a resistive impedance. Thereby, the capacitor does not smooth the edges but enables the edges to be coupled to the load as two very short duration bipolar pulses separated from each other by approximately ten microseconds. The two bipolar short duration pulses have frequently been of sufficient amplitude to destroy the load or have serious deleterious effects thereon, even with the insertion of a regulator between the converter and the load. Matching the impedance of the capacitor to that of the converter transformer inductance has been unsuccessfully attempted to obviate the problem.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for substantially decoupling DC loads from AC power line spikes which have a tendency to be coupled to the loads through AC to DC converters.
Another object of the present invention is to provide a new and improved circuit for decoupling DC loads from bipolar pulses induced across output terminals of an AC to DC converter including a step-down transformer and electrolytic capacitor.
Another object of the invention is to provide a new and improved circuit for substantially decoupling DC loads from bipolar pulses resulting from spikes on an AC line coupled through a stepdown transformer to an AC to DC converter rectifier bridge including an electrolytic output capacitor that is unable to function as a capacitor to frequency components in leading and trailing edges of the bipolar pulses.
A further object of the present invention is to provide a circuit for substantially decoupling pulses on an AC line from a DC load connected to the line by an AC to DC converter, wherein it is not necessary to provide an impedance match between the converter and the load.